Diene/nitrile polymers, commonly called nitrile rubbers or NBR rubbers, are well known. The polymers are cured using sulfur or peroxide cure systems to yield vulcanizates having excellent original physical properties and good oil resistance. However, the vulcanizates have unsatisfactory resistance to oxidation on thermal aging. The use of antioxidants and stabilizers in these polymers has not provided a totally satisfactory solution. Attempts have been made to improve heat resistance by the use of special cure systems such as the cadmium-magnesium oxide cure system disclosed in an article by D. A. Paulin in Rubber Age, October, 1969, page 69. Unfortunately, the cadmium compounds are toxic, and thereby special handling precautions must be taken. It would be highly advantageous to have a diene-nitrile rubber composition that, upon cure, yields a vulcanizate having as good as or better heat aged resistance than is achieved using a cadmium-magnesium oxide cure system.